Add Chromecast support

[derat]

I'd been dragging my feet on this since I don't have any idea how credential-passing works, and handing off Google credentials to a Chromecast device seems like a nightmare. Thinking about it more, though, maybe I could implement #3 and then have the web and Android clients pass signed URLs to Chromecast. I'd still need some additional way to temporarily authorize the Chromecast to report plays to the server, though.

[derat]

I'm not sure that this is worth doing. Most (all?) Google Home devices seem to also support Bluetooth, and I'm anxious about how buggy the cast experience is (especially around queue management) on first-party apps like the YouTube web and Android clients. Implementing a parallel "remote play" mode in all clients feels like it will be painful, too.

[derat]

I think I have yet to encounter a Chromecast app that works reliably. The Netflix Android app seems to disconnect from the Cast device periodically, resulting in a state where the only control I have over playback is stuff like pause and volume that's exposed by the Home app. I don't know how to convince Netflix to reconnect to the device when it's in this state.